Differential transmission lines are used in high-speed data communication in order to reduce the effect of electrical interference on the signal. A differential transmission line usually consists of a pair of wires, one positive and one negative. Ideally the signal propagation in the positive and negative wires is the same with respect to the shield or ground. This results in a signal pulse that has a minimum dispersion (growth in width).
FIG. 1 shows graphs of voltage vs. time for the positive and negative signals 102, 104. The positive signal 102 has a signal peak at time t=tp. The negative signal 104 has a signal peak at time t=tn. In this case, tp=tn. A differential signal 108 that results from the two signals running through a subtracter 106 has a small dispersion and large voltage peak.
FIG. 2 is similar to that shown in FIG. 1, except that the positive and negative signals 202, 204 have a skew with respect to each other. In this case, tp does not equal tn. The differential signal 208, as seen as the output of the subtracter 206, has a larger dispersion and lower peak voltage compared to that shown in FIG. 1. One skilled in the art will quickly recognize that the differential signal 108 in FIG. 1 is much more desirable than the differential signal 208 in FIG. 2 for reliable high-speed data communication. With long cable lengths (20 meters for example) and high data transmission rates (1.5 GHz for example), skew becomes a major issue.
Thus, there is a need for differential pair signal de-skewing in data communication systems.